1. Field of the Invention
The present invention relates to detecting elements for mainly measuring physical properties of fluids by utilizing conversion or transmission of heat.
2. Related Art Statement
For example, sensing detector elements are known to be used for measuring physical properties such as a flow amount, flow rate, temperature, etc. of intake air in internal combustion engines or the like.
One such detector element is shown in FIG. 5 for example. In this example, a Pt wire or a Pt film 34 is provided on the outer surface of a hollow pipe 31 made of a ceramic, and metallic lead wires 32A and 32B are fixed to opposed end openings of the hollow pipe 31 with a glass paste, a metallic paste, an organic adhesive or the like as denoted by 33A, 33B. An inorganic glass protective layer 35 is provided on the outer surfaces of the Pt wire or Pt film 34 and the hollow pipe 31.
This protective layer 35 is to prevent changes in resistance of the metallic resistor due to contamination with a fluid to be measured, and also to fix the metallic resistor to the surface of the hollow pipe. At the same time, when the surface of the protective layer is kept smooth, dust in the fluid to be measured is prevented from being attached and heaped up on the surface of the detecting element and sufficient strength is conferred upon the entire element. Therefore, the thickness of the protective layer must be greater than the su face roughness of the hollow pipe and the unevenness of the metallic resistor (wire, spiral thin layer or the like) so that influences of the surface roughness of the pipe and the unevenness of the metallic resistor upon the surface state of the protective layer may be controlled to a minimum and given smoothness may be maintained.
However, the above-mentioned detecting element utilizes heat exchange or heat transmission between the metallic resistor and the fluid to be measured. Therefore, if the thickness of the inorganic glass protective layer having great thermal insulation is increased as mentioned above, the heat transmission between the resistor and the fluid is interrupted, so that the response of the detecting element is remarkably lowered.